1. Field of Invention
This invention relates to current limiters, and particularly, but not exclusively, to current limiters for rectified power supplies.
2. Description of Related Art
FIG. 1 shows a rectifier circuit for converting an alternating current (a.c.) input into a rectified and smoothed direct current (d.c.) output. The circuit comprises a bridge rectifier 10 which produces a full-wave rectified output from the a.c. input which is applied across terminals 30. The full wave rectified output of rectifier 10 is smoothed by means of a capacitor 20 to provide the circuit output across terminals 40. In a practical application, a load requiring a d.c. input would be connected across the terminals 40.
When the a.c. supply is first switched on, the capacitor 20 is uncharged and a very high current is initially drawn as the capacitor charges. This in turn may damage the components of the circuit, particularly the diodes of the rectifier 10. In addition, the current surge reflected into the supply is often unacceptable. In order to prevent this, it has been proposed to connect a switch in the circuit between the rectifier and the capacitor, with a resistor in parallel with the switch. Such a circuit is shown in FIG. 2. The switch 70 is held open when the a.c. supply is switched on, and the rectified current is then forced to flow through the resistor. Thus, only a proportion of the total e.m.f. available is dropped across the capacitor 20, thereby limiting the amount of current drawn as the capacitor charges. After a period, the voltage across the capacitor rises to some suitable level as the capacitor charges and the switch can be closed. This process is commonly called `soft starting`. A modification of this well-known arrangement is shown in U.S. Pat. No. 5,087,871, which is incorporated herein by reference.
One problem with such a circuit is that if the capacitor 20 is faulty, the charge it stores may never be sufficient to create a significant voltage drop across it. Alternatively, the load to which the circuit output at the terminals 40 is connected may malfunction, causing abnormally large currents to be drawn and preventing the normal working voltage across the capacitor from being established. In either case, the high current drawn through the resistor 50 will not decrease sufficiently to allow the switch 70 to be closed. Significant current will then be drawn through the resistor 50 for a prolonged period, causing it to get very hot and at least pose a fire risk if not actually to catch fire itself.
A second problem is that the switch 70 may become stuck in the open circuit condition. This would also cause the normal load current to flow through the resistor for extended periods with the associated fire risk.
A further problem is that users of the equipment may cause the resistor 50 to overheat by repeatedly discharging the capacitor, then charging again using this circuit. The resistor 50 will typically be chosen to comfortably accommodate the energy dissipated in one charging cycle. However, it then requires a significant period of time to cool before the charging sequence can be repeated. Repeated cycling (e.g. by the user switching the input on and off too rapidly and repeatedly) may cause the resistor to overheat.
European published patent application number 667666, which is incorporated herein by reference, shows a circuit having detection means which detects if the switch is closed prior to switching on the a.c. supply to the rectifier 10. If it is, the circuit is prevented from operating. Although this arrangement precludes high start-up currents from occurring accidentally, it does not address the more serious problem which arises when, for example, the capacitor is unable to store charge properly or a load draws excessive current, nor when the switch is unable to close in the first place.
It is an object of the present invention to provide a circuit which at least alleviates these problems of the prior art.